In color matching where colors reproduced by an input device are matched with those reproduced by an output device, the colors reproduced by these devices are associated with each other on the basis of color reproduction characteristics of each device.
Generally, the color reproduction characteristics described above are calculated using a typical colorimeter illustrated in FIG. 18, in accordance with a procedure, such as that illustrated in FIG. 17. FIG. 17 illustrates an example of a procedure for calculating color reproduction characteristics of an image output device. First, a color chart is printed or output on a predetermined medium by the image output device. Then, spectral reflectance R(λ) of the output color chart is measured by a colorimeter. In the colorimeter illustrated in FIG. 18, first, a sample (color chart) is irradiated with light from a colorimetric illuminant included in the colorimeter. The irradiation light is reflected by the sample. A photodetector receives spectral radiance of the reflected light through a spectroscope. Dividing the received spectral radiance of the reflected light by spectral radiance of the illuminant gives the spectral reflectance R(λ) of the sample. Next, spectral radiance S(λ) of an illuminant (viewing illuminant) under which an output image is viewed is measured. Then, CIE tristimulus values XYZ are calculated from the measured spectral reflectance R(λ), the spectral radiance S(λ) of the viewing illuminant, and color-matching functions x(λ), y(λ), and z(λ) by using the following equations (1).
                    {                                                            X                =                                  k                  ⁢                                                            ∫                      380                      730                                        ⁢                                                                  R                        ⁡                                                  (                          λ                          )                                                                    ⁢                                              S                        ⁡                                                  (                          λ                          )                                                                    ⁢                                                                        x                          _                                                ⁡                                                  (                          λ                          )                                                                    ⁢                                              ⅆ                        λ                                                                                                                                                                    Y                =                                  k                  ⁢                                                            ∫                      380                      730                                        ⁢                                                                  R                        ⁡                                                  (                          λ                          )                                                                    ⁢                                              S                        ⁡                                                  (                          λ                          )                                                                    ⁢                                                                        y                          _                                                ⁡                                                  (                          λ                          )                                                                    ⁢                                              ⅆ                        λ                                                                                                                                                                    Z                =                                  k                  ⁢                                                            ∫                      380                      730                                        ⁢                                                                  R                        ⁡                                                  (                          λ                          )                                                                    ⁢                                              S                        ⁡                                                  (                          λ                          )                                                                    ⁢                                                                        z                          _                                                ⁡                                                  (                          λ                          )                                                                    ⁢                                              ⅆ                        λ                                                                                                                                                    equations        ⁢                                  ⁢                  (          1          )                    
However, if a medium or ink contains a material (e.g., fluorescent whitener) that includes a fluorescent component, the spectral reflectance of the image output device measured by the foregoing method may be different from spectral reflectance under the viewing illuminant. Here, the fluorescent component is a component by which light in an excitation wavelength region, which is a predetermined wavelength region of irradiation light, is reflected in a different wavelength region (i.e., emission wavelength region).
Referring to FIG. 19, a description will be given of measurement in which spectral reflectance of a sample containing a fluorescent component is measured by the colorimeter illustrated in FIG. 18. When a colorimeter, such as that illustrated in FIG. 18, is used to measure a sample containing a fluorescent component, light in an excitation wavelength region reacts to the fluorescent component in the sample and is reflected in an emission wavelength region. Therefore, the reflected light received by the colorimeter is one that is affected by the amount of fluorescence which is dependent on the spectral radiance of the colorimetric illuminant in the excitation wavelength region. That is, spectral reflectance is also dependent on the colorimetric illuminant. Thus, as illustrated in FIG. 20A, when a colorimetric illuminant and a viewing illuminant are the same, since spectral radiance of the viewing illuminant in the excitation wavelength region corresponds to the amount of fluorescence, correct XYZ values can be calculated. On the other hand, as illustrated in FIG. 20B, when the colorimetric illuminant and the viewing illuminant are different, since the spectral radiance of the viewing illuminant in the excitation wavelength region does not correspond to the amount of fluorescence, correct XYZ values cannot be calculated. In other words, if an illuminant used to measure a sample containing a fluorescent component is different from an illuminant in an actual viewing environment, XYZ values, which are colorimetric values, do not correspond to the actual appearance of the color.
Japanese Patent Laid-Open No. 2006-84333 describes a method in which spectral reflectance of a sample under a viewing illuminant that is different from a colorimetric illuminant is estimated by taking a fluorescent component into account. Specifically, in this method, the spectral reflectance of the sample is measured under two different illuminants. Then, the spectral reflectance of the sample under the viewing illuminant is calculated on the basis of a ratio, in each of the illuminants, between the sum of spectral radiances in an excitation wavelength region and the sum of spectral radiances in an emission wavelength region.
However, the amount of fluorescence is not determined by the sum of spectral radiances in the excitation wavelength region. Therefore, even if the amount of fluorescence is calculated on the basis of the ratio between the sums as described above, it is not possible to determine, with high accuracy, the spectral reflectance under any viewing illuminant.
In view of the points described above, the present invention aims to determine, with high accuracy, colors of a sample containing a fluorescent component under any viewing illuminant.